1. Field of the Invention
The present invention relates generally to an apparatus and method for purifying contaminated air, and more specifically to an apparatus and method for removing organic contaminants from an airstream by using adsorbent material.
2. Description of the Background Art
Volatile Organic Compounds (VOCs) are pervasive in today's technological society. VOCs are created as by-products in the electronics and other industries and may include chlorinated solvents, alcohol, esters, and acids. For VOCs that are released as contaminants into the atmosphere, a number of systems exist for their removal. These systems include fixed bed adsorbers and fluidized bed adsorbers. In both fixed and fluidized bed adsorption systems, the VOC-contaminated atmosphere or airstream is passed through a bed of adsorbent particles. The particles decontaminate the airstream by adsorbing the VOCs from the airstream. The VOCs are subsequently removed or desorbed from the particles in a manner that prevents the VOCs from being released back into the surrounding environment.
The most common adsorbent material has been activated carbon particles. Other adsorbent particles that may be used to clean a VOC-contaminated airstream or atmosphere include zeolite and polymeric adsorbents.
In typical fixed bed adsorber systems, the VOC-contaminated atmosphere passes through canisters containing adsorbent particles. The VOC-contaminated airstream enters one end of the canister by way of a conduit and passes through a bed of adsorbent particles. The VOCs in the contaminated airstream are deposited onto the adsorbent particles by adsorption or absorption, or a combination of both. The decontaminated airstream then exits the canister by way of a second conduit typically located at a second end of the canister.
Fixed bed adsorber systems are expensive to maintain. The containers in the fixed bed adsorber systems must be periodically removed so that the adsorbent particles in the containers may be desorbed after the particles become saturated with VOCs. Fixed bed adsorption systems also have the disadvantage that the adsorbent particles maintain contact with each other, thus reducing the exposed surface area of each particle that is available for adsorption of VOCs. Additionally, in forcing the contaminated atmosphere through the canister, the packed nature of the adsorbent particles requires fixed bed adsorption systems to provide a significant amount of air pressure.
In fluidized bed adsorption systems, the VOC-contaminated atmosphere is passed through adsorbent particles resting on perforated trays. The perforations allow VOC-contaminated air to pass through the trays without allowing the adsorbent particles to fall through the holes. As the velocity rate of the contaminated air passing through the bed is increased, the adsorbent particles separate slightly from each other and expose more of their surfaces to the airstream. At some point, the air velocity causes the adsorbent particles to act as a fluid, and to form localized bubbles that appear as if the particles are boiling. The dynamic movements of the bubbles increase the surface area of an adsorbent particle that will be exposed to the VOC-contaminated air flow. U.S. Pat. No. 4,902,311, issued on Feb. 20, 1990, discloses an apparatus having an adsorption chamber with fluidized beds, and a desorption chamber coupled to the adsorption chamber. U.S. Pat. No. 4,902,311 is fully incorporated herein by reference thereto as if repeated verbatim hereinafter.
The fluidized bed adsorption method and system decrease the air pressure required to move the contaminated airstream through the adsorbent and has better contact efficiency than a fixed bed adsorber. However, as in fixed bed adsorption systems, the entire surface area of the adsorbent particles is not exposed to the contaminated atmosphere. Since only the surfaces of the bubbles are exposed, the amount of VOCs adsorbed onto the particles and removed from the VOC-contaminated air is limited. Another problem with fluidized bed systems is that two or more beds must be used so that the bubbles created in the first bed are broken by the next bed in order to expose more of the contaminated air to the adsorbent particles.
In both fixed and fluidized bed adsorption systems, when the adsorbent particles become saturated with VOCs, the adsorbent particles require replacement or reconditioning. In a typical VOC reconditioning, the VOCs are desorbed by subjecting the VOC-saturated adsorbent particles to heat. As with adsorption of VOCs from a VOC-contaminated atmosphere, the adsorbent particles are in a packed state during the desorption process. This packed state limits the amount of surface area of an adsorbent particle that is exposed to the heat, thereby reducing the amount of VOCs desorbed from an adsorbent particle.
VOCs may be removed from a VOC-contaminated airstream by causing the adsorbent particles to become entrained in the VOC-contaminated airstream. By causing the adsorbent particles to be entrained, a surface of an adsorbent particle is separated from the surface of the other adsorbent particles. Thus by causing the adsorbent particles to be entrained, the surface areas of the adsorbent particles can have about 100% contact with the VOCs in the VOC-contaminated airstream. To permit the surface areas of the adsorbent particles to be fully exposed to the VOC-contaminated airstream results in an efficient adsorption method. Other adsorber and desorber systems are discussed in commonly assigned U.S. Pat. No. 5,538,541, issued on Jul. 23, 1996 and U.S. patent application Ser. No. 08/644,327, filed on May 10, 1996, now U.S. Pat. No. 5,667,559, all of which are fully incorporated herein by reference thereto as if repeated verbatim hereinafter.
Thus, what is needed and what has been invented is an apparatus and method that overcome these problems and increase the amount of VOCs removed from a VOC-contaminated atmosphere.